The present invention relates to a frequency detector for detecting a singal which has been frequency-modulated by a digital signal and, more particularly, to a frequency detector suitable for an integrated circuit.
Implementations heretofore proposed for accomplishing a frequency detector in an integrated circuit configuration includes a so-called direct conversion system. An advantage attainable with the direct conversion system is that it directly converts a received carrier wave to a baseband signal so as to allow filtering and other processings to be performed in the baseband. Signal processing in the baseband may be implemented by a well known method which relies on differentiation and multiplication. Although the differentiation and multiplication scheme advantageously demodulates even a signal which has been modulated by an analog signal, great difficulty is experienced in realizing a circuit for practicing the method due to the need for gain control and balance of the circuitry particular thereto.
Concerning prior art constructions of the kind concerned, a reference may be made to U.S. Pat. No. 4,322,851 (Vance) which shows in FIG. 4 a decoding logic circuit arrangement for an FSK (frequency shift keyed) receiver. The arrangement comprises two inverters, four NOR gates, two SR flip-flops, four D flip-flops, and two resistors. The problem with such an arrangement is that as many as 140 transistors are necessary in total in order to implement the circuit elements mentioned above, particularly all the gate elements, rendering the overall circuit bulky and, therefore, unsuitable for an integrated circuit. Meanwhile, U.K. Patent Application GB 2 106 359 A (Standard Telephones and Cables Limited) shows in FIG. 11a a demodulator construction furnished with an analog circuit section which includes a switch, resistors, a low pass filter and other elements. While the analog circuit section of the illustrated demodulator may be removed to produce a demodulated digital signal, spike noise is apt to appear in the output of the demodulator because of a delay difference between two baseband signals applied to an output, that is, an exclusive OR circuit.